i2c: mux: demux-pinctrl: check initial mux selection, too

[ Upstream commit ca89f73394daf92779ddaa37b42956f4953f3941 ]

When misconfigured, the initial setup of the current mux channel can
fail, too. It must be checked as well.

Fixes: 50a5ba87 ("i2c: mux: demux-pinctrl: add driver")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>